The industrial use of solvents such as chlorinated solvents, glycols and glycol ethers is being increasingly called into question because of the toxicity of these products and their negative effect on the environment. There is therefore a search to substitute them by “green” solvents, namely biodegradable and nontoxic solvents, such as lactic acid esters and in particular ethyl lactate. The latter exhibits a number of physicochemical properties which allow it, at least potentially, to substitute for all or some of the conventional solvents and, inter alia:                an excellent solvating power for resins (nitrocellulose, acrylic resins, polyurethanes, polyesters, alkyds, epoxy, and the like),        a relatively high solubility in water and a generally good solubility in the majority of organic solvents,        a relatively low volatility, allowing, for example, its effective use in surface treatment applications,        a relatively high boiling point (154° C.), allowing its effective use in applications involving high temperatures.        
The solvent properties of ethyl lactate and other lactic acid esters can advantageously be taken advantage of in numerous fields of activity, such as the metal industries, the automobile and aviation industries, the paint and ink industries, the resin and varnish industries, the pharmaceutical and cosmetological industries, and the semiconductor industries.
It should be specified that the notion of solvent is in this instance in no way limiting and includes, in particular, the notions of cosolvent, cleaning, degreasing or stripping agent.
These lactic acid esters, in particular ethyl lactate or butyl lactate, have, however, other industrial uses as synthetic intermediates, in particular in the preparation:                of lactic acid of very high purity, as disclosed, for example, in U.S. Pat. No. 5,210,296 and EP 614 983, or,        of lactide, i.e. of cyclic dimer of lactic acid, and then of polylactides, as disclosed for example, in patent WO 93/15127.        
Despite its undeniable advantage as solvent or synthetic intermediate, a product such as ethyl lactate remains to this day a product which is still not manufactured and used industrially to any great extent, in particular with regard to chlorinated solvents, glycols or glycol ethers. This is because of its price which, by way of example, is currently still two to four times higher than that of the main commercially available glycol ethers.
It is widely known that the difficulty in preparing ethyl lactate industrially at an economically acceptable cost is related in particular to the twofold technical constraint 1) of having to continuously remove the water present in the reaction medium for the purposes of shifting the equilibrium of said reaction in the desired direction and 2) of having to employ amounts of alcohol, in this instance of ethanol, which are in large excess in comparison with those theoretically necessary for the reaction for the esterification of lactic acid.
This constraint is all the more pronounced because ethanol exhibits, in comparison with other alcohols, such as butanol, an azeotrope which is low in water, rendering the removal of the water necessarily expensive.
In fact, some technologies for the preparation of lactic acid esters are regarded as inapplicable to the specific production of ethyl lactate. This is the case, for example, with the processes exemplified in the abovementioned U.S. Pat. No. 5,210,296 and EP 614 983, disclosing the preparation of butyl lactate after employing n-butanol in a culture medium, optionally concentrated and/or purified, having generated ammonium lactate.
According to the authors of these patents, alcohols comprising 4 to 5 carbon atoms are those, among the alcohols which can be used for esterifying lactic acid, which exhibit the best overall economic compromise in terms of yields of formation and then of purification of esters but also of optional subsequent hydrolysis of said esters for the purpose of recovering lactic acid of high purity. n-Butanol is thus disclosed in U.S. Pat. No. 5,210,296 as exhibiting the following technical/economic advantages:                boiling point (117.7° C.) very significantly greater than that of water and very significantly lower than that of butyl lactate (187° C.),        low miscibility with water and possibility of forming heterogeneous azeotropes with water,        possibility of being distilled without giving rise to problems a) of polymerization or dimerization of butyl lactate, b) of hydrolysis of butyl lactate and c) of increase in the viscosity of the reaction medium.        
It has recently been envisaged, in U.S. Pat. No. 5,723,639, to improve the abovementioned processes for the preparation of lactic acid esters from ammonium lactate and in particular to be able to render said processes truly applicable to the preparation of ethyl lactate and/or to the use of limited reaction temperatures, i.e. lower than 100° C. Said patent claims, for this purpose, the necessary use of a stage of pervaporation of the esterification medium for the purpose of removing therefrom both water and ammonia, this being achieved without concomitant removal of the alcohol (ethanol, butanol) or of the ester formed (ethyl lactate or butyl lactate).
However, this technology requires the use of relatively specific and expensive pervaporation devices and in particular of pervaporation membranes which have to exhibit, but in particular to retain over time, very specific characteristics of selectivity and of resistance to the conditions of the medium (temperatures of between 75 and 150° C., presence of acidic or alkaline compounds, and the like).
In addition, while the possibility of operating at relatively low temperatures (80-95° C. for example) makes it possible to limit the level of ethanol employed to a value of 2 mol/mole of lactic acid, as described in example 6 and 7 of said patent, this possibility still in practice has a number of harmful effects on the general economics of the process and in particular a) relatively high levels of introduction of esterification catalysts (generally from 1 to 10%) and/or b) very long reaction times (generally of several tens of hours) and/or c) (very) low degrees of conversion of lactic acid (or ammonium lactate) to ethyl lactate.
Provision has also been made to prepare ethyl lactate not from lactic acid or one of its salts but from very specific compounds, such as the lactide, as disclosed in the summary of Japanese Patent No. 8-40983, or polylactides of high molecular weight (200 000), as disclosed in example 10 of U.S. Pat. No. 5,264,617.
Such processes are not economically viable since they use starting materials (lactide, polylactides) which are even more expensive than the desired product (ethyl lactate). This is because, as has already been emphasized, ethyl lactate is itself used for the purpose of the preparation of lactide and polylactides and it is economically inappropriate to wish to reconvert these products to ethyl lactate, a compound with a lower added value.
In addition, according to example 10 of U.S. Pat. No. 5,264,617, the degree of conversion of polylactide to ethyl lactate remains limited since it only achieves a value of 78%.
The result of the above is that there exists a need to have available a simple and efficient means which is less expensive than the abovementioned ones for obtaining lactic acid esters and in particular ethyl lactate, which means make it possible in particular:                to use purified or unpurified lactic acid as starting material,        to limit the technical and economic problems related to the presence of water in and to the removal of water from the esterification medium,        to limit the technical and economic problems related to the use of alcohols, in particular of ethanol, in the esterification medium and to the recovery of alcohols, in particular of ethanol, from the esterification medium,        to esterify lactic acid at relatively high temperatures in relatively short reaction times and/or without having to overdose the esterification catalysts, and        to prepare said esters, in particular ethyl lactate, with high degrees of conversion, i.e. greater than 85%, this being achieved without employing specific, complex or expensive plant.        